Recovery of cooking liquor from spent semi-chemical pulping liquors



RECQVERY F CQQKING LIQUOR FROM SPENT SEMLCHEMICAL 'PULPING LIQUORS Eugene W. Schoeffel, Kronenwetter, Wis., assignor, by

mesne assignments, to SteriingDrug Inca, New York, N. Y., a corporation of New York Application February 21, M52, Seriai N0. 272,719

Claims. (Cl. 92-2) No Drawing.

The present invention relates to the semi-chemical pulping of Wood and is more particularly concerned with acid or. other sulfite or sulfate procedure.

However, no technically operative means have been discovered for theeconomic reclamation of the sodium sulfite cooking liquor from thespent semi-chemical pulp liquor. Conventionally the alkali regeneration has been accomplished by concentration of the Waste liquor, followed bya smelting or cooking process in which the sodium sulfate andsodiumsulfiteare converted primarily to soduim sulfide (Naz'S) and sodium carbonate M12003 The generated hydrogen sulfide is expelled with carbon dioxide and subsequent treatment with a mixture ofcarbon dioxide. and sulfur dioxide'is employed to depress the formation of elemental sulfur and thiosulfate or ploythionate salts. The hydrogen sulfide (H28) is then converted to sulfur dioxide and thevreclaimed cooking liquor formed-from the sodium carbonate and sulfur dioxide, eliminating sodiumthiosulfate (NaaSzOs) formation. Maximum yields of the recovered sodium ion of slightly over seventy percent have been reported, large sodalosses being incurred during the smelting or coking phase of the recovery. process. T o date,no simple procedure for the. regeneration of the semi-chemical pulping liquor (NazSOa) has been demonstrated to be .economically feasible.

Therefore, it is an object of the present invention to provide a simple and economical method for the recovery of cooking liquor from the semichemical pulp waste liquors in good yields.

It is another object of the present invention to provide a method for the substantially complete recovery and regeneration of the sodium from spent semichemical pulp liquor.

ltis still a further objectof the present invention to provide ajprocess for the recovery .ofsodium sulfite cooking liquor from spent semi-chemical pulp liquor by aqueous, liquid phase, flameless oxidation of .the spent semi-chemicalpulp liquor and conversion of .the thusformed sodium salts in .good yield to sodium sulfite (NazSOa) and sodium hydroxide ,(NaOH).

Another object of'the present invention is to provide a method for the regeneration of the spent semi-chemical pulp liquor wherein the "cooking liquor iscontinuously recovered from the Waste pulp liquor :in a properly bufieredsodium sulfite"solution"ready'to be -returned to atent in situ.

the pulping phase of the cycle, thereby obviating the 4 procedures.

Afurther object of the present invention is to provide a method for the recovery of semi-chemical pulping liquor that insuresa continuous supply of.regenerated cooking liquor that can immediately be utilized in the pulping phase of the semi-chemical pulping cycle.

Other objects of the presentv invention will be apparent to one skilled in the art to which this invention pertains. The novel process of the present invention is directed to. the recovery of sodium sulfite from spent semi-chemical pulp liquor and comprises the steps of oxidizing substantially completely the said liquor while maintained under the vapor pressure of the reaction mixture and substantially in the liquid phase in a reaction zone, introducing sulfur dioxide (S02) and a calcium alkali into the sodium-containing concentrate from .said reaction zone, separating the calcium sulfate thus precipitated, treating the resultingfiltrate with a calcium alkali, and returning the sodium sulfite (Naz'SOs) so-formed to the pulping, process and the calcium sulfite so-formed to the said sodium-containing concentrate. In the aqueous, liquid-phase, flameless oxidation of the spent pulp liquor, the inorganic salts are changedto their highest states of oxidationand the sulfurous .materials aresubstantially completely converted into sodium sulfate (NazSOt), virtuallyall of the remaining sodium ion being converted to sodium carbonate (NazCOs). The concentration of the'liquor during the oxidation step enhances the subsequent successive conversion of the sodium sulfate to sodium vbisulfite and thence to the desired sodium sufite. The amounts-of sulfur lost fromthe system as calcium sulfateare at the-expense of economically reclaiming substantially all of the-sodium ion. Careful regulation of the addition of calcium alkalies in the conversion of sodium bisulfite to sodium sulfite is productive of an already buffered regenerated cooking liquor having the .desired'pH in .accord with the precise type of the semichemical pulping process beingconducted.

The process of the present invention has utility in the economical regeneration .of the valuable sodium suliite pulping liquor from the spentsemi-chemical pulp liquor by a procedure wherein the recovered cooking liquor is immediately recycled to the pulping process, thereby permitting a virtually continuous pulpingoperation in which :the overall sodium-loss is small. vRecovery of a small amount of the sodium ion as sodium hydroxide,

associated with-the sodium sulfite, enables-the buffering action of the sodium hydroxide :to be advantageously employed in the regenerated cooking liquor. amounts of the calcium sulfite (CaSOa) formed in the liming of the .sodium bisulfite are returned to the phase Useful of the cycle wherein the sodium sulfate (Na2SO4) is reacted with calcium bisulfite Ca(HSO3)2), .produced The S04 ion vis eliminated .as calcium sulfate, which under controlled roasting conditions yields the necessary sulfur dioxide employedin the treatment of sodium-containing concentrate .obtained by oxidation of the wasteliquors.

In carrying out the process of the present invention, a starting spent liquor or effluent, resulting from the semichemical pulping of soft or hard woods, or mixtures thereof, with acid, neutral, or alkaline sodium sulfite in the presence of sodium carbonate, sodium hydroxide or sodium bicarbonate, and even in the presence of some sodium sulfate and sodium thiosulfate, is oxidized. Any waste liquor or effluent wherein substantial amounts of sodium sulfite have been employed and are present per se or in the form of sodium sulfate or sodium thiosulfate is an appropriate starting material from which the desired sodium sulfite can be regenerated according to the process of the present invention.

In substantially completely oxidizing the spent semichemical pulp liquor, the necessary apparatus comprises a pump for continuously charging a reactor with spent semi-chemical pulp liquor, an air compressor, a tower reactor provided with means therein to remove periodically any precipitate formed therein, and a flash chamber to receive the oxidized residual spent semi-chemical pulp liquor from the reactor.

In initiating the oxidation phase, the spent semi-chemical pulp liquor is pumped into the reactor under pressure of from about 200 to 2000 pounds per square inch, the preferred pressure being that which is sufficient to maintain substantially all of the waste efiluent in the liquid phase. The waste liquor charge is then heated by means of an oil ring to a temperature of between above 100 degrees and 330 degrees centigrade, temperatures of from 235 to 250 degrees centigrade being useful for sodium sulfate recovery. Compressed air, or other compressed gaseous materials capable of furnishing free oxygen, under a pressure slightly greater than the pressure in the reaction zone is then introduced into the reactor containing the spent semi-chemical pulp liquor through a dispersion head to initiate the oxidation of the combustible materials. The rate of charging the reactor with spent semi-chemical pulp liquor and the rate of introduction of the compressed air or other oxidation media is syn chronized to insure the substantially complete oxidation of all the inorganic and organic constituents.

Once initiated, the oxidation proceeds exothermically so that no external heating is required and, in fact, the spent pulp liquor can be introduced into the reactor at a temperature of as low as twenty degrees centigrade, the exothermic heat liberated from the oxidation being sufiicient to raise the temperature of the carbonaceous material in the incoming spent chemical liquor to a point where substantially complete oxidation will ensue. The quantity of oxidizing agent supplied is preferably that theoretically required to convert all of the organic combustible material to its ultimate end products, viz carbon dioxide (CO2) and water (H20), and in general sulficieut oxygen to produce substantially complete oxidation of all other oxidizable materials in the spent pulp liquor should be employed in the oxidation reactor.

The oxidized liquor from the reactor is then continuously passed through a flash chamber, from which the fixed gases, i. e., nitrogen, carbon dioxide, and excess air are vented off with steam under pressure. The liquid efiluent is then obtained from the reactor.

The liquid effluent (sodium-containing concentrate) from the flash chamber contained primarily sodium sulfate (NazSOq), a smaller amount of sodium carbonate (NaCOa) and a certain amount of sodium acetate and acetic materials. In this concentrate, virtually all (greater than 99 percent) of the sulfur materials and most of the sodium are present as sodium sulfate, the remaining minor amounts of sodium ion being contained in the sodium carbonate and sodium acetate.

The thus-oxidized, spent semi-chemical pulp liquor is next reacted with calcium bisulfite (Ca(SOaH)2) to yield chiefly a precipitated calcium sulfate (CaSOr) and an aqueous acidic sodium bisulfite (NaHSO3) solution. In such reaction, the calcium bisulfite can be added per se to the sodium-containing concentrate or added by forming calcium bisulfite in situ by reacting sulfur dioxide with a calcium alkali such as calcium carbonate, calcium oxide, calcium hydroxide, calcium sulfite (CaSOs) and the like in the presence of water. The in situ preparation of the calcium bisulfite is preferred since the calcium sulfite produced as a by-product in the subsequent liming of the sodium bisulfite can be continuously recycled into the system at this point and admixed with the continuous flow of sulfur dioxide to form the requisite calcium bisulfite reagent. The calcium bisulfite reaction step is generally conducted in a closed system in order to reduce corrosion of the equipment to a minimum. The reaction mixture comprising the sodium-containing concentrate and the added calcium alkali is maintained at about room temperature or slightly above, temperatures of l2-25 degrees centigrade being preferred, and the preferred sulfur dioxide gas introduced into the system over a period of one or more hours. The course of the reaction and degree of completion thereof can be gauged by employing a sufficient analytical control, e. g., a carbon dioxide absorption tube, to avoid the use of an excess of sulfur dioxide. The pH of the sodium bisulfite solution is controlled in part by the amount of sulfur dioxide utilized in the conversion of sodium sulfate to sodium bisulfite and is preferably less than five, the pH value of between about three and 4.5 being regarded as useful. Rapid conversion of the sodium sulfate to sodium bisulfite is achieved by the employment of vigorous, continual stirring of the reaction admixture. Substantially in excess of ninety percent of the sodium sulfate is converted to sodium bisulfite and a like percentage of calcium alkali is converted to calcium sulfate. If desired, the calcium sulfate precipitate can be filtered and removed from the cycle at this point, or, alternatively, it can be kept in admixture with the acidic, aqueous, sodium bisulfite solution and removed at a subsequent point in the regeneration cycle.

The aqueous, acidic sodium bisulfite (NaHSOs) solution is next treated With a calcium alkali, resulting in the formation of the desired, regenerated sodium sulfite (NazSOs) together with a minor amount of sodium hydroxide (NaOl-I). Control of the added alkali regulates the final pH of the regenerated sodium sulfite cooking liquor, thereby facilitating the recovery of a cooking liquor that is thus buffered to the desired degree and which can immediately be recycled and utilized in the pulping of fresh quantities of Wood. Calcium carbonate is the preferred alkali embodiment, although calcium? oxide (lime), calcium hydroxide and the like are entirely operative in the aqueous liquid medium. The conversion of thesodium bisulfite to sodium sulfite is conducted at slightly elevated temperatures, seventy degrees centigrade being preferred. The calcium alkali may be introduced into the reaction mixture in a single addition, or, alternatively, may be added in small portions during the course of the reaction, e. g., from about fifteen minutes to about three hours or more. Sufficient quantities of alkali are added to insure the formation of an alkaline admixture, pH values of from seven to twelve being generally satisfactory. When lime (calcium oxide) is reacted with sodium bisulfite solution, a pH of about twelve of the aqueous sodium sulfite reaction product is obtainable. Employment of a comparable amount of calcium carbonate (CaCOa) instead of calcium oxide may result in the formation of an admixture having a pH of about seven to nine. If necessary, nitrogen may be continuously circulated through the reaction system to prevent oxidation and resultant formation of undesirable oxidized inorganic products. Conversion of sodium bisulfite (NaHSOs) to sodium sulfite (NaSOa) in amounts substantially above ninety percent is consistently achieved, a varying amount of sodium hydroxide, e. g., one to eight or nine percent, can be formed in the reaction product. The sodium hydroxide exhibits a desirable buffering effect on the resewage generated sodium sulfite'cooking liquor. Upon completion of the reaction, the admixture is filtered, the filter cake comprising primarily the precipitated calcium sulfite (CaSOs) which is subsequently returned to the sodiumcontaining concentrate obtained fromzthe oxidation of the spent semi-chemical pulp liquor and is reacted with. sulfur dioxide in the presence of water to form-insitu the .calcium bisulfite reagent. The aqueous, alkaline filtrate contains chiefly the desired sodium'sulfite cooking liquor and the desired amount of sodium hydroxide. Since the pH of the filtrate has been controlled by the alkaliaddition to the sodium bisulfite, the regenerated'cooking liquor is thus adequately buffered for the precise type of semichemical pulping process being conducted and can be continually recycled intothe: fresh pulpingapparatus.

The following examples illustrate the practice of the present invention but are not'to be construedas limiting the same.

Example 1 Spent semi-chemical pulp liquor fromthe semi-chemical pulping of a mixture of soft and hard woods with a buffered sodium sulfite cookingliquor was oxidized substantially completely in a system comprising a pump for continuously charging the reactor with spent pulp liquor, an air compressor, a tower reactor and a flash chamber to receive the oxidized residual waste liquor-from the reactor and from which the'effluent gases are vented. The spent semi-chemical pulp liquor was pumped into the tower reactor under a pressure of about 800 pounds per square inch and the waste liquor charge'heated by means of an oil ring at a temperature of 235 degrees centigrade. Compressed air under pressure slightly greater thanthat in the reaction zone was then introduced intothereactor through a dispersion head to initiate the oxidation of the combustible materials, and thereafter the aqueous, liquid phase, flameless oxidation was'self-sustaining, requiring no further external heat,'the rate of chargingthe reactor with spent semi-chemical pulp liquor and the'rate of introduction of compressed air being regulated to insure substantially complete oxidation of all of the organic and inorganic constituents. The oxidized liquor from the tower reactor was continuously passed through a flash chamber from which nitrogen, carbon dioxide and excess air were vented off with steam at about '800 pounds per square inch pressure. The liquid efiluent was continuously drained from the flashchamber. Analysis of an appropriate sample of the resulting sodium-containing concentrate showed the entire sulfur content and the major portion of the sodium ion to be present as sodium sulfate (NazSOr), the concentrate also containing minor amounts of sodium carbonate, sodium acetate and acidic materials. An aliquot portion of the thus-oxidized spent semi-chemical pulp liquor containing five-tenths mole of sodium sulfate was admixed with five-tenths mole of calcium carbonate and the resulting mixture diluted to a volume of about 500 milliliters with water, placed in a three-necked, one-liter round bottom flask, equipped with a sealed stirrer, a gas inlet tube leading to the bottom of the fiask, an outlet tube and reflux condenser connected with a carbon dioxide absorption vessel. While the temperature of the reaction mixture in the flaskwas maintained at 23 degrees centigrade, sulfur dioxide (S02) gas was introduced into the system over a period of about one hour. When the carbon dioxide tube had absorbed 26.0 grams (theoretical 22.0 grams) of carbon dioxide, the apparatus was disconnected, the resulting mixture filtered and the filter cake Washed with five successive ZOO-milliliter portions of water. Analysis of an appropriate sample indicated that over ninety percent of the sodium sulfate was converted to sodium bisulfite (NaHSOs) and-that over ninety percent of the calcium carbonate was converted to calcium sulfate (CaSOr). The aqueous, acidic sodium bisulfite filtrate was returned to.the. reaction fflask, warmed to seventy degrees centigrade and 36 grams of 9510: percent calcium oxide (CaO) added inismall portions. Theresulting mixture was stirred for three hours, at the endof which time the pH was 12.4. Analysis of the completed reaction product showedthat 69.2 percent of the originalxsodium from the. starting sodium-containing concentrate was present in final form as sodium sulfite (NazSO3 7.1 percent of the sodium-as sodiumhy- .droxide (NaOH), which together with the approximately three'percent sodium contained inthe sodium bisulfite solution removed for analysis 'earlier:represented a total conversion yield of about eighty percent. An additional sixteen percent .of the original sodium ion was present as sodium sulfate (Na2SO4), apparently having been formed either from the oxidation of the sodium bisulfite or the sodium sulfite (Naz'SOs). Thus, about 96 percent of the original sodium ion was recovered. The reaction mixture was filtered, the filtrate containing the desired regenerated sodium 's'ulfite and sodium hydroxide cookingliquonwhereasthe filter cake comprised a mixture of calcium sulfate and calcium sulfite. The residue contained 52.2 percent calcium sulfite (CaSOz).

Example 2 Employing the apparatus'for converting sodium sulfate to sodium bisulfite, illustrated in Example 1, the residual calcium suliite ('CaSOs) obtained upon completion of the reactions i'nExample 1 was introduced into the reaction flask along with another aliquot of oxidized semichemical liquor, prepared according to the procedure of Example 1 and containing one-half mole of sodium sul- 'fate, and sulfur dioxide gas admitted to'the system at a temperature of seventy degrees centigrade. A stream ofnitrogen was introduced to forcibly expel the air from the.reaction system. After an excess of sulfur dioxide had been taken up in the absorptiontube, the mixture was 'filtered and a sample of the filtrate assayed as in Example '1. Direct conversion of the sodium in the sodium-containing concentrate to sodium bisulfite was greater than ninety percent. The aqueous sodium bisulfite (NaHSOs) filtrate-was returned to the reaction flask, excess calcium oxide (0210) introduced at seventy degrees centigrade and'the contents of the flask vigorously agitated for two hours, nitrogen being continuously circulated through the system to prevent oxidation. At the end of the twohour period, the pH of the reaction admixture was 11.1.

Filtering'of the flask contents yielded 83 percent of the starting sodium ion as sodium sulfite (NazSOs), 1.6 percent as sodium hydroxide (NaOH), which together with the three percent removed for bisulfite analysis in thefirst step represented about 87 percent conversion to regenerated semi-chemical pulping liquor.

Example 3 Spent semi-chemical pulp liquor was oxidized under aqueous, liquid phase conditions at a temperature of 250 degrees centigrade and pressure of 400 pounds, employing the apparatus and procedure described in Example 1,the oxidized liquor containing primarily sodium sulfate of calcium sulfate (virtually a quantitative yield), whereas the sodium bisulfite (NaHSOa) filtrate analyzed at 6.4 parts sulfur dioxide content (corresponding to a yield ofapproximately 10.4 parts of NaHSOa). To the aqueous sodium bisulfite solution was added 5.6 parts of calcium'oxideiand the resulting mixture heated by means of .a;water;bath for .one hour with vigorous stirring, the

pH of the admixture at the end of this period being twelve. The admixture was filtered, the desired filtrate containing primarily the sodium sulfite (NazSOs) cooking liquor with a small amount of sodium hydroxide. The filtered residue comprised calcium sulfite and a minor amount of calcium sulfate. The sulfur dioxide (S02) content of the recovered semi-chemical sodium sulfite cooking liquor was 3.6 grams per 100 milliliters, indicating that one-half of the sulfur dioxide had been converted into the precipitated calcium sulfite. The total sodium recovery based upon the soda content of the starting oxidized, spent semi-chemical pulp liquor was about 93 percent.

Example 4 Following the procedure of Example 2, another aliquot of sodium-containing concentrate obtained by the oxidation procedure of Example 1 and having 14.2 parts of sodium sulfate, and the calcium sulfite residue from the process of Example 3, were admixed and vigorously agitatcd with sulfur dioxide in the manner of Example 3. Subsequent filtration removed the precipitated calcium sulfate and the residual aqueous sodium bisulfite filtrate was stirred in a closed system for a period of about two hours with an excess of calcium carbonate, instead of the calcium oxide employed in Example 3. A final pH of 9.2 was obtained. The product was filtered, yielding the precipitated calcium sulfite and the aqueous sodium sulfite cooking liquor. Overall recovery of the sodium ion was about 91 percent based upon the sodium sulfate input. The precipitated calcium sulfite was again recycled into the oxidized spent pulp liquor and the regenerated semi-chemical cooking liquor employed in pulping fresh quantities of wood.

The novel process of the present invention achieves an overall regeneration and recovery yield of the sodium ion, based upon the soda content of the completely oxidized spent semi-chemical pulp liquor, of from about 85 to 98 percent. The overall calcium and sulfate ion recovery is generally in excess of 98 percent. The reaction by-products from the various individual steps of the regeneration procedure are re-cycled and employed in other phases of the recovery process. The precipitated calcium sulfate, either separated after completion of the reaction in which the sodium bisulfite (NaHSOs) is formed or separated after final conversion of the sodium bisulfite to the regenerated cooking liquor, can be roasted at a temperature of approximately 1600 degrees centigrade to convert a minimum of 90 percent of the total sulfur content into the desired sulfur dioxide, which in turn is recycled into the agitating sodium-containing concentrate of the oxidized spent semi-chemical pulp liquor to form the calcium bisulfite reagent and to control the acidity of the resulting sodium bisulfite solution. The remaining calcium oxide (lime) from the roasting procedure can then be conveniently employed in the subsequent reaction wherein the sodium bisulfite is converted to the regenerated sodium sulfite cooking liquor.

Various modifications may be made in the invention without departing from the spirit or scope thereof and it is to be understood that I limit myself only as defined in the appended claims.

I claim:

1. In a process for the recovery of sodium sulfite from spent semi-chemical pulp liquor, the steps comprising: pulping wood chips with a buffered solution of sodium sulfite; separating pulp and a spent semi-chemical pulp liquor; oxidizing substantially completely the organic and inorganic constituents of said liquor while maintainedv under the vapor pressure of the reaction mixture and substantially in the liquid phase in a reaction zone to obtain a sodium-containing concentrate; introducing sulfur dioxide and a solube calcium-containing material which will form insoluble calcium sulfate when contacted with an aqueous solution of sodium sulfate; separating calcium sulfate thus precipitated and treating the resulting filtrate with a calcium-containing material which will form sodium sulfite and calcium sulfite with said filtrate; separating the calcium sulfite so-formed and returning it to said sodium-containing concentrate; and returning the sodium sulfite so-formed to the pulping step.

2. In a process for the recovery of sodium sulfite from spent semi-chemical pulp liquor, the steps comprising: pulping wood chips with a buffered solution of sodium sulfite; separating pulp and a spent semi-chemical pulp liquor; oxidizing substantially completely the organic and inorganic constituents of said liquor while maintained under the vapor pressure of the reaction mixture and substantially in the liquid phase in a reaction zone to obtain a sodium-containing concentrate; introducing sulfur dioxide and calcium carbonate into said sodium-containing concentrate from said reaction zone; separating the calcium sulfate precipitated and treating the resulting filtrate with an excess of calcium oxide to form sodium sulfite and calcium sulfite; separating the calcium sulfite so-formed and returning it to said sodium-containing concentrate; and returning the sodium sulfite so-formed to the pulping step.

3. In a process for the recovery of sodium sulfite from spent semi-chemical pulp liquor, the steps comprising: pulping wood chips with a buffered solution of sodium sulfite; separating pulp and a spent semi-chemical pulp liquor; oxidizing substantially completely the organic and inorganic constituents of said liquor while maintained under the vapor pressure of the reaction mixture and substantially in the liquid phase in a reaction zone to obtain a sodium-containing concentrate; introducing calcium bisulfite into said sodium-containing concentrate from said reaction zone; separating the calcium sulfate thus precipitated and treating the resulting filtrate with an excess of a calcium-containing material which will form sodium sulfite and calcium sulfite; separating the calcium sulfite; and returning the sodium sulfite so-formed to the pulping step.

4. In a process for the recovery of sodium sulfite from spent semi-chemical pulp liquor, the steps comprising: pulping wood chips with a buffered solution of sodium sulfite; separating pulp and a spent semi-chemical pulp liquor; oxidizing substantially completely the organic and inorganic constituents of said liquor while maintained under the vapor pressure of the reaction mixture and substantially in the liquid phase in a reaction zone to obtain a sodium-containing concentrate; introducing sulfur dioxide and a soluble calcium-containing material which will form insoluble calcium sulfate when contacted with an aqueous solution of sodium sulfate; treating the resulting filtrate with a calcium-containing material which will form sodium sulfite and calcium sulfite; separating the calcium sulfite so-formed and returning it to said sodiumcontaining concentrate; and returning the sodium sulfite so-formed to the pulping step.

5. In a process for the recovery of sodium sulfite from spent semi-chemical pulp liquor, the steps compris ng: pulping wood chips with a buffered solution of sodium sulfite; separating pulp and a spent semi-chemical pulp liquor; oxidizing substantially completely the orgamcand inorganic constituents of said liquor while maintained under the vapor pressure of the reaction mixture and substantially in the liquid phase in a reaction zone to obtain a sodium-containing concentrate; introducing sulfur dioxide and a calcium-containing material selected from the group consisting of calcium oxide, calcium carbonate; calcium hydroxide, calcium bisulfite and calcium sulfite; separating calcium sulfate thus-formed and treating the resulting percipitate with a calcium-containing material selected from the group consisting of calcium carbonate, calcium oxide, and calcium hydroxide, separating the calcium sulfite thus-formed and returning it to said sodium- 9 10 containing concentrate; and returning the sodium sulfite 2,516,827 Marshall et a1 July 25, 1950 so-formed to the pulping step. 2,528,350 Farber Oct. 31, 1950 2,547,913 Lagally Apr. 3, 1951 References Cited in the file of this patent 2,598,311 Schoeffel May 27, 1952 UNITED STATES PATENTS 5 2,6 2,291 Bry n 1954 1,765,560 Barbou June 24, 1930 FOREIGN PATENTS 1,815,328 Richter y 1931 27,733 Denmark June 6, 1921 1,983,789 Bradley et al. Dec. 11, 1934 2,010,077 Haglund Aug. 6, 1935 10 OTHER REFERENCES 2,029,616 Haglund Feb. 4, 1936 Paper Trade Journal, pages 30-34; October 12, 1950, 2,354,553 Sherk July 25, 1944 Oxidation of Sulphate Black Liquor, T. T. Collins.

2,482,594 Pearl et a1. Sept. 20, 1949 

1. IN A PROCESS FOR THE RECOVERY OF SODIUM SULFITE FROM SPENT SEMI-CHEMICAL PULP LIQUOR, THE STEPS COMPRISING: PULPING WOOD CHIPS WITH A BUFFERED SOLUTION OF SODIUM SULFITE; SEPARATING PULP AND A SPENT SEMI-CHEMICAL PULP LIQUOR; OXIDIZING SUBSTANTIALLY COMPLETELY THE ORGANIC AND INORGANIC CONSTITUENTS OF SAID LIQUOR WHILE MAINTAINED UNDER THE VAPOR PRESSURE OF THE REACTION MIXTURE AND SUBSTANTIALLY IN THE LIQUID PHASE IN A REACTION ZONE TO OBTAIN A SODIUM-CONTAINING CONCENTRATE; INTRODUCING SULFUR DIOXIDE AND A SOLUBLE CALCIUM-CONTAINING MATERIAL WHICH WILL FROM INSOLUBLE CALCIUM SULFATE WHEN CONTACTED WITH AN AQUEOUS SOLUTION OF SODIUM SULFATE; SEPARATING CALCIUM SULFATE THUS PRECIPITATED AND TREATING THE RESULTING FILTRATE WITH A CALCIUM-CONTAINING MATERIAL WHICH WILL FORM SODIUM SULFITE AND CALCIUM SULFITE WITH SAID FILTRATE; SEPARATING THE CALCIUM SULFITE SO-FORMED AND RETURNING IT TO SAID SODIUM-CONTAINING CONCENTRATE; AND RETURNING THE SODIUM SULFITE SO-FORMED TO THE PULPING STEP. 